I'm confused by transformation matrices. How do I do this? Please help!

You're expected to be familiar with the various standard transformation matrices, including the one for rotation in an anti-clockwise direction by an angle .

So I suggest you look up the rotation matrix in your notes or textbook and then determine the angle that gives this particular matrix.

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We haven't taken notes on this, nor do we have a textbook. (We use Exeter, which is a bunch of random problems thrown together and bound.)

Rotation matrix - Wikipedia, the free encyclopedia

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